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Egyptian Journal for Aquaculture
P-ISSN: 2090-7877
E-ISSN: 2636-3984
www.eja.journals.ekb.eg/
Walaa et al., 2020; 10 (2):35-45
DOI: 10.21608/eja.2020.94283
Marine gelatinous zooplankton of the Egyptian waters: a
review
Walaa S. Zaghloul; Fedekar F. Madkour ; Samya H. Mohammad
Marine Science Department, Faculty of Science, Port Said University
Correspondence: E-mail: cathren_100@yahoo.com
Received: Nov. 25, 2018; Accepted: May.20, 2020 published: 2020 Vol.10 (2):35-45
Abstract
Gelatinous plankton is formed by representatives of
cnidarians (true jellyfish), ctenophores (comb jellies) and
tunicates (salps). Gelatinous groups are a conspicuous, but
relatively little studied component of marine ecosystems; in
recent years its importance in pelagic food webs has gained
increased awareness. They are essential components of the
marine food web, relaying primary production from
microbial algae to fish and seabirds. Gelatinous zooplankton
may cause substantial mortality of fish eggs and larvae. This
may play an important role in decreasing fishing stock of
commercial fisheries, and they are an important variable in
fisheries science and that it cannot be overlooked. The
objective of this paper is to review a current knowledge on
gelatinous zooplankton in the Egyptian waters of
Mediterranean and Red Seas.
Key words: Gelatinous zooplankton, Jellyfish, Cnidarians, Ctenophores,
Red Sea, Mediterranean Sea.
1. Background
Jellyfish is the descriptive common name first attributed in the mid-
1800s to various medusae and since the 1990s also considered to include
the ctenophores and siphonophores on the basis of atrophic equivalence
(Mills, 1995). The popular name "jellyfish" reflects the prominence of
the mesoglea. The name has no accurate zoological definition, and
though the two super classes Cubomedusae and Scyphomedusae, might
be considered the "true" jellyfish. The name is sometimes applied also to
hydromedusae, siphonophores, and some other gelatinous plankton such
as ctenophores and slaps. The great benefit of the term its figurative
35Walaa et al., 2020; Egy. J. Aquac 10(2):35-45
simplicity however has led to confusion as ‘jellyfish’ also has been
applied to other gelatinous aquatic animals (Brotz et al., 2012).The
problem is that occasional broader usage can imply functional or other
equivalence that does not exist, increase ambiguity about the attributes of
jellyfishes and blur potentially important distinctions from other
gelatinous aquatic animals already known collectively as gelatinous
zooplankton (Hamner & Dawson, 2009).
Gelatinous macro zooplankton, consist of Cnidaria (true jellyfish),
Ctenophora (comb jellies) and Chordata (thaliaceans or pelagic
tunicates). The representatives of these three phyla are grouped together
because of their gelatinous bodies constituting the bulk of gelatinous
macro zoo-plankton and, together, make up what we call “jellyfish”
(Boero et al., 2008). These diverse species nonetheless differ in their
evolutionary histories and may have distinct morphologies, life histories,
ecologies and other traits (Duarte et al., 2014).The modern popular
perception that gelatinous zooplankton are position most uniquely among
metazoan so take advantage of future global change (Mills, 1995) raises
the following question; In what circumstances is being gelatinous
beneficial in the plankton? First, the evolution of a planktonic lifestyle
and gelatinous body plan could be driven by ecological benefits derived
from feeding, distribution, predator avoidance (e.g., being invisible), or
breaking cycles of parasitism (Hamner & Dawson, 2009).
The fossil record tells us that true jellyfish are the oldest animals
among those that are still living today, being represented in fossils that
date back to the Pre-Cambrian; they are referred to the phylum Cnidaria
(Cartwright et al., 2007).Gelatinous species range from the surface to
great depths in every sea, and in sizes from microns to meters. Gelatinous
zooplankton are widely heralded as key members of ocean ecosystems,
yet their ecological, roles are often grossly oversimplified and
misunderstood. Often they are too fragile and damaged to record or
identify in traditional plankton nets and trawls, or they are simply
ignored.
The following sections contain a knowledge account of important
phyla, summarizing the information that isrelevant for the scopes of this
review.
36Walaa et al., 2020; Egy. J. Aquac 10(2):35-45
1.1. Cnidarians
The true jellyfish are the planktonic stages of three cnidarian classes:
the Hydrozoa, the Scyphozoa, and the Cubozoa. Most Scyphozoa and all
Cubozoa fall within the category of macro- and even mega zooplankton,
since they are large enough, as adults, to be perceived by the naked eye,
ranging from 2 mm (e.g. some small medusae) to 2 m in bell diameter,
and several meters of tentacle length, of the largest medusae. Some
Hydrozoa are macroplankters too, but many species belong to the
mesozooplankton, being smaller than 2 mm. Gelatinous
mesozooplankton is usually not perceived by a casual observer, unless
when its representatives reach high densities (Boero, 2013).
Cnidaria is an ancient and diverse phylum containing 10,211 species
worldwide and 752 species in the Mediterranean Sea inhabiting
exclusively aquatic and mostly marine environments (Coll et al., 2010;
Appeltanset al., 2012). Many cnidarians are distributed in shallow waters
as they are in a symbiotic relationship with endosymbiotic algae for food
and calcification (Muscatine & Porter, 1977). Cnidarians do have
stinging cells armed with cnidocysts; they move by jet propulsion,
contracting their bells or umbrellas. The umbrella usually carries
tentacles on its margin and has a manubrium hanging in its cavity. The
mouth lies at the end of the manubrium. The tentacles catch the prey and
bring it to the manubrium (Boero, 2013).
The diversity of the hydromedusan fauna augments between 500 and
800 m depth (Kramp, 1968). Hydromedusae are among the main
predators of the ocean and constitute a significant but often
underestimated constituent of the pelagic deep fauna. Data about their
composition, distribution, trophic relations and relationships with other
oceanic strata are very scarce, and are based on limited observations.
They seem to play a much more important role in oceanic transfer of
energy than previously thought. The knowledge of the deep-sea
Hydroidomedusae fauna is still incomplete and biased evenfor the polyp
stage. The depth and vertical distribution of hydromedusae are often
uncertain owing to the sampling methodology, generally only the upper
limit oftheir vertical distribution is known and it is practically impossible
from literature to distinguish between meso- and bathypelagic species.
Furthermore, the sampling with plankton nets, even at discrete depths,
damage or break up most of the delicate specieswhich then cannot be
properly identified (Boero, 2013).
37Walaa et al., 2020; Egy. J. Aquac 10(2):35-45
Jellyfish exhibit reproductive and life history strategies (e.g., single-
parent reproduction, high reproductive capacity, short maturation times)
that allow them to quickly reach “bloom concentrations” (“blooming” is
here defined as suddenly and rapidly occurring en masse) in the pelagic
phase when environmental conditions are favorable (Hamner & Dawson,
2009).The controlling predatory influence jellyfish blooms can exert on
zooplankton populations often results in cascading effects throughout the
local food web (Burrell & Engel, 1976). In addition, jellyfish blooms can
have detrimental economic effects on fisheries, sting swimmers, and clog
intakes of power and water desalinization plants (Purcell et al., 2007;
Purcell, 2012).
1.2. Ctenophores
Ctenophores are a small, well defined phylum of bi-radially
symmetrical animals with over 200 currently known species distributed
worldwide. They are exclusively marine, planktonic or benthic includes 7
orders, and comprise a significant portion of the planktonic biomass in
their range (Appeltans et al., 2012). They are solitary organisms with
gelatinous soft bodies. The general body plan is closely to cnidarian, but
more advanced than that of Cnidaria (Harbison & Arai, 1986).
Gelatinous macro zooplankton is usually equated to stinging jellyfish,
and its presence causes major concern about own safety in non-marine
biologists, due to fear of potential stings. Many members of gelatinous
zooplankton, however, are not cnidarians, and do not sting. The
ctenophores do not have a bell and a manubrium, and do not move by
pulsations, they just share a gelatinous appearance with the cnidarians.
Ctenophores move by ciliary propulsion, through what zoologists call
“ctenes” or combs; hence the popular name: comb jellies (Boero, 2013).
They can be a few centimeters, or even 50 or more centimeters, being
globular, or similar to a dirigible, or ribbon like. Ribbon like ones, of the
genus Cestum, can move also by snake like movements, but the other
members of the group usually glide, appearing motionless and, in spite of
that, moving. Their bodies are characterized by iridescent glows that are
caused just by the flapping combs, the propulsors of the animal (Boero,
2013).
In coastal waters, ctenophores can reach great abundances. They feed
at high rates on zooplankton and ichthyoplankton, and thereby may be
detrimental to fish populations (Purcell & Arai, 2001). Ctenophores are
well adapted to utilize favorable food conditions, such as patches of
38Walaa et al., 2020; Egy. J. Aquac 10(2):35-45
zooplankton, with feeding rates not saturating until at very high food
levels (Greene et al., 1986). Due to their high fecundity and rapid growth
potential (Reeve & Walter, 1978), they can quickly increase their
abundance and predation rates and thereby act as key organisms in
marine pelagic ecosystems by controlling the density of prey (Purcell &
Arai, 2001).Ctenophores are among the most difficult marine animals to
study, and represent the greatest challenge to conventional oceanographic
sampling standard (Harbison & Miller, 1986).
1.3. Chordata (Subphylum Tunicata)
Pelagic tunicates comprise the Thaliacea and the Larvacea, or
Appendicularians which belong to subphylum Tunicata, members of the
phylum Chordata. The Larvacea are of small size, but can be present in
very high quantities. The Thaliacea, namely salps, doliolids and
pyrosomes, are of much larger size. Pyrosome are colonial forms while
salps form chains reaching several meters in length. Pelagic tunicates are
much different from both Cnidaria and Ctenophora in their feeding
habits; they are filter feeders upon protists (usually phytoplankton),
bacteria and even viruses. Their life cycles are holoplanktonic and
involve both sexual and asexual reproduction, with the possibility of high
biomass increases due to formation of large colonies. Apparently, just as
for ctenophore, the pelagic tunicates do not have benthic stages (Boero,
2013).
2. Gelatinous zooplankton in the Egyptian Red Sea
The Red Sea, also known as Bahr al Ahmar in Arabic, is a semi-
enclosed, elongated warm body of water about 2,000 km long with a
maximum width of 355 km, a surface area of roughly 458,620 km2, and a
volume of 250,000 km3(Head, 1987). The Red Sea includes about 3.8 %
of the world's coral reefs. The Red Sea is bounded by nine countries and
incised by numerous coastal lagoons and a large number of islands and
extensive groups of shoals, and at its northern end is bifurcated by the
Sinai Peninsula into the Gulf of Aqaba and the Gulf of Suez (Head, 1978;
Najeeb and Ian, 2015)
Although gelatinous zooplankton have an essential role in marine food
chains, relatively few systematic seasonal studies have been conducted in
the Red Sea. Red Sea is poor in gelatinous zooplankton. Dowidar (2003)
recorded that gelatinous zooplankton represent about 2% of the total
zooplankton density. Recently, Zaghloul (2017) revealed higher
39Walaa et al., 2020; Egy. J. Aquac 10(2):35-45
abundance of gelatinous zooplankton recorded 22% of the overall
zooplankton density. Only26 siphonophore species were reported by
Halim's review (1969). Dowidar (2003) recorded 29 cnidarian species
(16 Hydromedusae 11 Siphonophore, and 2 Scyphomedusae) and one
ctenophore species in the northern Red Sea and Gulf of Aqaba. Zaghloul
(2017) reported 24 species of them 18 belong to Hydromedusae, 3 are
Scyphomedusae and 3 species belong to phylum Ctenophora. In the Red
Sea, seasonal bloom of Scyphomedusae, Aurelia aurita, Siphonophore,
Diyphs chamissonis, and Hydromedusa, Aglaura hemistoma have been
documented (Alamaru et al., 2009; Zaghloul 2017). The previous studies
on the gelatinous zooplankton in the Red Sea reported 75 Hydromedusae
species, six Scyphomedusae species and one species of ctenophores.
Zaghloul (2017) recorded 13 species as a new record of gelatinous
zooplankton community in the Egyptian Red Sea to increase the whole
species number in Red Sea to be 85 Hydromedusae species, seven
Scyphomedusae species and three species of Ctenophora.
3. Gelatinous zooplankton in the Egyptian Mediterranean Sea
The Mediterranean Sea is located between Europe, Asia, and Africa.
Excluding the Black Sea, it covers 2,542,000 Km2, with an average depth
close to 1,500 m and a maximal depth of 5,121 m in the Ionian Sea. The
Mediterranean Sea is an almost closed basin, connected with the Atlantic
Ocean via the Strait of Gibraltar, with the Red Sea via the Suez Canal,
and with the Black Sea via the Bosphorous and the Dardanelles Straits.
Surface temperatures range between 11-13°C in winter and 25-30°C in
summer, determining cold temperate to warm-temperate conditions in the
cold season, and tropical conditions in the warm one. Deep-water
temperature is about 13°C and, normally, this is also the surface
temperature in winter, when the basin becomes homoeothermic. Summer
thermoclines divide the variable surface waters from the more stable,
deep waters (Bouillon et al., 2004).
Despite the ecological importance of the gelatinous marine
zooplankton in the marine food web (Purcell & Kremer, 1983),
knowledge of the gelatinous zooplankton in the Egyptian Mediterranean
waters is far from complete. The previous studies on zooplankton
population in the Egyptian Mediterranean waters have tended to focus
mainly on copepods, the major zooplankton component. Other groups
such as siphonophores and hydromedusae were regarded as being of
secondary importance in terms of numerical abundance and hence not
40Walaa et al., 2020; Egy. J. Aquac 10(2):35-45
treated in detail (Dowidar& El-Maghraby, 1973). Dowidar and El-
Maghraby (1970) added six species of hydromedusae from the coast off
Alexandria.
Pelagic cnidarians constitute a permanent component of the
holoplankton in the south eastern Mediterranean. Hussein (1977) found
that siphonophores constitute about 85% by number of cnidarians while
Hydrozoa constitute about 2.6% of the average density of all zooplankton
in the Egyptian Mediterranean waters. Dowidar (1981) performed a
qualitative and quantitative investigation on the pelagic coelenterates of
the Egyptian Mediterranean waters. The ecology and distribution of
major zooplankton groups other than copepods, in particular
hydromedusae, siphonophores, amphipods, decapods and pelagic
tunicates occurring in the Mediterranean shelf waters off the Egyptian
coast were studied by Zakaria (1992) within the framework of the project
"Biological productivity of the South eastern Mediterranean in the post-
High-Dam period".
4. Gelatinous zooplankton bloom and migration
By the pulsed nature of their life cycles, gelatinous zooplankton come
and go seasonally, giving rise in even the most undisturbed
circumstances to summer blooms. Even holoplanktonic species like
jellyfish and ctenophores increase in number in the spring or summer
when planktonic food is available in greater abundance. Beyond that
basic life cycle-driven seasonal change in numbers, several other kinds of
events appear to be increasing the numbers of jellies present in some
ecosystems (Purcell et al., 2007). Jellyfish blooms are a conspicuous
feature in the oceans. The problem of gelatinous zooplankton blooming
in the Mediterranean and Red Seas remains complex.
Gelatinous zooplankton distribution and migration are affected by
both biotic and abiotic factors. The climate variability and anthropogenic
activities have great impacts on community composition. The most
factors are represented by increasing water temperatures and on the other
side, the ballast water regarded as the main factor affecting the
introduction of non-indigenous aquatic organisms. A ballast water tank
canthus function as an incubator during the cruise for some species. This
fact may have some impact on the release of non-indigenous species in
the destination harbor or in near coastal waters where the ballast tanks
are emptied (Purcell et al., 2007).
41Walaa et al., 2020; Egy. J. Aquac 10(2):35-45
The blooming and migration of gelatinous zooplankton in the
Egyptian waters in both seas may have both positive and negative
outcomes on the marine ecosystem of the Mediterranean and Red Sea
depending on the species. The positive impact is enriching the
biodiversity of both seas as the well diversified ecosystem become more
stable and healthier. The negative impact happens when the occurrence
of alien species might threat the diversity or abundance of the native
species of both seas. Another negative impact is concerned with the
human activities, health and/or economic interest
Monitoring program is needed to record the recent erythrean
gelatinous species in the Egyptian waters and follow up the distribution
and abundance of those previously recorded as alien species in the
Egyptian Mediterranean and Red Sea waters to assess their impacts on
the native biodiversity of both seas. The monitoring program must be
accompanied with studies on the feeding habits of these gelatinous
species to detect the role of these species in the food web and to
understanding how gelatinous zooplankton populations will behave
because these are the most predator of fish larva and fish egg, In this
way, a more advanced technique should be developed for identification
such as DNA analysis to reach the species level.
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